This invention relates to a liquid crystal display (LCD) having an increased pixel aperture ratio and different colored polymer filters. More particularly, this invention relates to a liquid crystal display including an array of TFTs wherein photo-imageable color filters having contact vias or apertures disposed therein are located on the active substrate between the address lines and pixel electrodes so that the pixel electrodes and color filters may both be permitted to overlap at least one of the row and column address lines without exposing the system to capacitive cross-talk, thereby providing an efficient and commercially improved high aperture LCD.
Electronic matrix arrays find considerable application in X-ray image sensors and active matrix liquid crystal displays (AMLCDs). Such AMLCDs generally include X and Y (or row and column) address lines which are horizontally and vertically spaced apart and cross at an angle to one another thereby forming a plurality of crossover points. Associated with each crossover point is an element (e.g. pixel) to be selectively addressed. These elements in many instances are liquid crystal display pixels or alternatively the memory cells or pixels of an electronically adjustable memory array or X-ray sensor array.
Typically, a switching or isolation device such as a diode or thin-film transistor (TFT) is associated with each array element or pixel. The isolation devices permit the individual pixels to be selectively addressed by the application of suitable potentials between respective pairs of the X and Y address lines. Thus, the TFTs act as switching elements for energizing or otherwise addressing corresponding pixel electrodes.
Amorphous silicon (a-Si) TFTs have found wide usage for isolation devices in liquid crystal display (LCD) arrays. Structurally, TFTs generally include substantially co-planar source and drain electrodes, a thin-film semiconductor material (e.g. a-Si) disposed between the source and drain electrodes, and a gate electrode in proximity to the semiconductor but electrically insulated therefrom by a gate insulator. Current flow through the TFT between the source and drain is controlled by the application of voltage to the gate electrode. The voltage to the gate electrode produces an electric field which accumulates a charged region near the semiconductor-gate insulator interface. This charged region forms a current conducting channel in the semiconductor through which current is conducted. Thus, by controlling the voltage to the gate and drain electrodes, the pixels of an AMLCD may be switched on and off in a known manner.
Typically, pixel aperture ratios (i.e. pixel openings) in non-overlapping AMLCDs are only about 50% or less. As a result, either display luminance is limited or backlight power consumption is excessive, thereby precluding or limiting use in certain applications. Thus, it is known in the art that it is desirable to increase the pixel aperture ratio or pixel opening size of LCDs to as high a value as possible so as to circumvent these problems. The higher the pixel aperture ratio (or pixel opening size) of a display, for example, the higher the display transmission. Thus, by increasing the pixel aperture ratio of a display, transmission may be increased using the same backlight power, or alternatively, the backlight power consumption may be reduced while maintaining the same display luminance.
It is known to overlap pixel electrodes over address lines in order to increase the pixel aperture ratio. For example, xe2x80x9cHigh-Aperture TFT Array Structuresxe2x80x9d by K. Suzuki discusses an LCD having an ITO shield plane configuration having a pixel aperture ratio of 40% and pixel electrodes which overlap signal bus lines. An ITO pattern in Suzuki located between the pixel electrodes and the signal lines functions as a ground plane so as to reduce coupling capacitance between the signal lines and the pixel electrode. Unfortunately, it is not always desirable to have a shield electrode disposed along the length of the signal lines as in Suzuki due to production and cost considerations. The disposition of the shield layer as described by Suzuki requires extra processing steps and thus presents yield problems. Accordingly, there exists a need in the art for a color LCD with an increased pixel aperture ratio which does not require an ITO shield plane structure to be disposed between the signal lines and pixel electrode.
It is old and well-known to make TFT arrays for LCDs wherein address lines and overlapping pixel electrodes are insulated from one another by an insulating layer. For example, see U.S. Pat. Nos. 5,055,899; 5,182,620; 5,414,547; 5,426,523; 5,446,562; 5,453,857; and 5,457,553.
U.S. Pat. No. 5,182,620 discloses an AMLCD including pixel electrodes which at least partially overlay the address lines and additional capacitor lines thereby achieving a larger numerical aperture for the display. The pixel electrodes are insulated from the address lines which they overlap by an insulating layer formed of silicon oxide or silicon nitride. Unfortunately, the method of making this display as well as the resulting structure are less than desirable because: (i) chemical vapor deposition (CVD) is required to deposit the silicon oxide or silicon nitride insulating film; (ii) silicon oxide and silicon nitride are not photo-imageable (i.e. contact holes or vias must be formed in such insulating layers by way of etching); and/or (iii) the dielectric constants of these materials are too high and thereby render the LCD susceptible to cross-talk problems. Still further, the ""620 patent does not discuss or contemplate color filter issues. As a result of these problems, the manufacturing process is both expensive and requires more steps than would be otherwise desirable. For example, in order to etch the contact holes in an insulating layer, an additional photoresist coating step is required and the user must be concerned about layers underneath the insulating layer during etching. With respect to CVD, this is a deposition process requiring expensive equipment. Furthermore, if the color filters are on the passive substrate, as they typically are, alignment of the active and passive plates is difficult and requires expensive equipment and expertise.
In the prior art, color filters in active matrix liquid crystal displays (AMLCDs) are typically located on the substrate (the passive plate or substrate) which opposes the active matrix substrate. In other words, the color filters and TFTs (or diodes) are typically located on different substrates, on opposite sides of the liquid crystal (LC) layer [e.g. see U.S. Pat. No. 5,499,126]. Black matrix formation is also typically provided on the color filter substrate. The provisions of the black matrix and color filters on the substrate opposite the active matrix is, of course, costly, time consuming, and requires numerous manufacturing steps. As discussed above, this also requires difficult and time consuming alignment steps.
The LCD structure disclosed in U.S. Pat. No. 5,641,974 utilizes a transparent polymer insulating layer on the active substrate to provide isolation between address lines and overlapping pixel electrodes. While this design work well and achieves superior results, it unfortunately, in practice, requires each of: (i) providing the transparent polymer insulating layer on the active substrate; (ii) providing color filters on the opposite substrate; (iii) providing a black matrix on the color filter substrate; (iv) very accurate plate-to-plate (i.e. substrate-to-substrate) alignment; and (v) the process steps required for (ii)-(iv) above.
It is apparent from the above that there exists a need in the art for an improved high aperture AMLCD design, and method of manufacturing same, which (i) reduces the number of total manufacturing process steps required; (ii) eliminates the need for the combination of (a) the optically transparent insulating layer sandwiched between the address lines and pixel electrodes, and (b) the color filters; (iii) reduces the need for the black matrix on the substrate opposite the active substrate; (iv) provides for a high pixel aperture ratio; and/or (v) reduces the accuracy required in plate-to-plate alignment (i.e. eliminates the need for sophisticated alignment machines).
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.
Generally speaking, this invention fulfills the above-described needs in the art by providing a high aperture color LCD including color filters, the display comprising:
first and second substrates;
a liquid crystal layer sandwiched between the first and second substrates;
first and second different colored pixels, said first pixel including on said first substrate a first pixel electrode, a first insulating color filter, and a first thin film transistor (TFT), and said second pixel including on the first substrate a second pixel electrode, a second insulating color filter, and a second TFT, wherein said first and second color filters are differently colored;
the first and second pixel electrodes overlapping with corresponding address lines in communication with TFTs so as to define a high aperture display, the overlapping forming areas of overlap;
the first insulating color filter being at least partially disposed in an area of overlap in the first pixel, the first color filter having a dielectric constant of less than about 5.0 and having a first contact hole defined therein that allows the first pixel electrode to be electrically connected to the first TFT; and
said second insulating color filter being at least partially disposed in an area of overlap in the second pixel, the second color filter having a dielectric constant less than about 5.0 and having a second contact hole defined therein that allows the second pixel electrode to be electrically connected to the second TFT.
In certain embodiments, each of the first and second color filters are of a photo-imageable material that includes a color dye or pigment.
In certain embodiments, the LCD includes arrays of only two colored pixels, while in other embodiments the display may include arrays of three differently colored pixels, or four differently colored pixels.
Surprisingly, it has been found that the thickness of the metal pixel electrode layers (e.g. ITO) should be from about 300 xc3x85-900 xc3x85(preferably about 600 xc3x85) in this invention in order to reduce the interface stress between the pixel electrodes (e.g. ITO) and the color filters.
This invention further fulfills the above-described needs in the art by providing a method of making a color LCD having insulating color filters, the method comprising the steps of:
providing first and second substrates;
providing a liquid crystal material;
forming an array of isolation switching elements on the first substrate and a plurality of address lines in communication with the isolation switching elements;
depositing a first resist color filter layer on the first substrate over top of the address lines and the switching elements;
photo-imaging the first resist color filter layer so as to pattern it into a first array on the first substrate so that color filters in the first array are of a first color and overlap at least a portion of at least one address line;
depositing a second resist color filter layer of a second color over top of the first array of color filters;
photo-imaging the second resist color filter layer so as to pattern it into a second array so that color filters in the second array overlap at least a portion of at least one address line;
forming contact holes in color filters in each of the first and second arrays;
depositing a conductive pixel electrode layer over top of the first and second arrays of color filters; and
patterning the electrode layer so as to form an array of substantially transparent pixel electrodes wherein pixel electrodes in the array overlap address lines which are also overlapped by color filters so that the color filters act as insulators between the pixel electrodes and the address lines in the areas of overlap, wherein each of the pixel electrodes is in electrical communication with a corresponding switching element through one of the contact holes.
In certain preferred embodiments, each of the color filters has a dielectric constant less than or equal to about 4.0 so as to reduce cross-talk and coupling capacitance in the areas of overlap.